The present invention relates generally to electrical reference voltage circuits and more particularly to methods and apparatus for reducing output voltage ripple in bandgap reference circuits.
Reference voltages are used in a wide variety of analog circuits, including wireless communications devices, memory devices, voltage regulators, conversion circuits, and others, to provide steady DC reference voltages. The reference voltages are used for biasing various circuit components, providing references to comparator circuit inputs, and for calibration circuits, and the like. For instance, in designing various analog circuits, such as digital to analog converters, voltage regulators, or low drift amplifiers, it is necessary to establish an independent, stable bias reference. Typically this reference is a voltage, which provides a substantially constant output voltage regardless of changes in input voltage, output current, or temperature, although current references are sometimes used.
Voltage reference circuits are sometimes designed using reference diodes such as Zener diodes, where a reference voltage is established across a biased diode and buffered for use in other circuitry. Over the past several decades, however, so-called xe2x80x9cbandgapxe2x80x9d voltage reference circuits have been predominantly used rather than Zener diode type approaches, due to superior reference voltage stability with changing temperature. Bandgap voltage reference circuits take advantage of temperature coefficients associated with semiconductor device physical properties so as to provide a reference voltage generally insensitive to thermal variations, at least with respect to first order effects. Such bandgap circuits are well known and many variations are in use, for example, in which second order non-linear effects are addressed or otherwise compensated for in providing stable reference voltages. The physical characteristics of semiconductor devices used to implement bandgap circuit designs are derived from the voltage gap between the conduction band and the valence band of the semiconductor material (e.g., silicon), and hence the term xe2x80x9cbandgapxe2x80x9d reference.
In a bandgap reference circuit, a signal corresponding to a base-emitter voltage (VBE) is summed with a signal corresponding to the difference in base-emitter voltages of two diode-connected transistors (xcex94VBE) of different emitter sizes in producing a bandgap output reference voltage. The first component VBE is known to have a negative temperature coefficient, whereas the latter component xcex94VBE has a positive temperature coefficient. Thus, the bandgap type reference circuit utilizes predictable temperature drift properties of opposite polarities with appropriate scaling, by which the effects of the two opposite-polarity drifts are made to cancel, resulting in a nominally zero temperature coefficient output voltage level.
In a bipolar transistor, the temperature dependence of the base-emitter voltage drop VBE exhibits a negative temperature coefficient of about xe2x88x922 mV per degree C. Conversely, the temperature dependence of xcex94VBE between two transistors is proportional to the absolute temperature through the thermal voltage VT, with VT equal to kT/q, where k is Boltzmann""s constant, T is the absolute temperature in degrees Kelvin, and q is the electron charge. The xcex94VBE term accordingly exhibits a positive temperature coefficient, and is sometimes referred to as a Proportional T o Absolute Temperature (PTAT) component. In typical bandgap circuits, one or both of these components, usually voltage signals, are scaled and the scaled signals are then subtracted in order to provide a temperature independent bias voltage, with the opposite polarity temperature coefficients canceling one another. In this manner, bandgap reference circuits compensate the negative temperature coefficient of a bipolar transistor""s base-emitter voltage, VBE, with the positive temperature coefficient of the thermal voltage VT associated with the difference in base-emitter voltages of two diode-connected transistors xcex94VB.
In addition to being temperature independent, voltage reference circuits should also provide a substantially constant output voltage in the presence of changing supply voltage levels and/or changing loading conditions. In this regard, the basic bandgap reference circuit designs suffer from output noise or ripple voltages caused by ripple or noise components in the power source supplying the bandgap circuit. One measure of the ability of a reference circuit to suppress or reject such supply ripple voltages is referred to as the power supply ripple rejection (PSRR). Within the context of modem high-speed digital devices, noise immunity or suppression is becoming more and more important, where fast switching of digital circuitry (e.g., in wireless communications and/or portable computational devices) may impart noise onto a supply voltage (e.g., such as a battery) providing power to the voltage reference circuit. Cascode devices are sometimes added to bandgap circuits to increase the PSRR (e.g., by reducing the amount of output ripple). However, cascode devices, if so employed, must be connected in series with other reference circuit components, between the supply voltage and ground. As a result, such cascode techniques reduce the voltage headroom available in the circuit as a whole. Another approach is to provide a pre-regulated power supply for the bandgap circuit. However, the circuit associated with the pre-regulation will consume more power, area and increase the complexity of the whole system.
In this regard, there is a continuing trend toward low power, low voltage systems, for example, such as wireless communications devices, portable computational devices, and the like, in which stable reference voltage circuits are needed. For instance, many modem wireless systems are being designed for operation using batteries supplying as low as 1.3 volts DC. In such applications, therefore, ripple reduction techniques involving cascode circuitry may be impractical or unworkable, such as where the bandgap reference output voltage is about 1.2 volts DC. Thus, there is a need for improved bandgap voltage reference circuits and techniques by which output ripple can be reduced without adversely impacting current and future supply voltage headroom requirements. Furthermore, as the power consumption constraints become more stringent, it is also desirable to provide reference circuits, such as bandgap systems with improved noise immunity, without significantly increased power consumption.
The following presents a simplified summary in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended neither to identify key or critical elements of the invention nor to delineate the scope of the invention. Rather, the primary purpose of the summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The invention involves reducing output ripple voltages in bandgap voltage reference circuit using ripple rejection circuitry. The ripple rejection circuitry represents a subtractor that has two input control signals (the first one is from the output of an error amplifier and the second one is from the power supply). The output of the ripple rejection circuitry is simply the subtraction of these two control signals. The inclusion of the supply voltage component in the control signal advantageously provides for improved power supply ripple rejection (PSRR). The ripple rejection circuitry, moreover, does not adversely affect the supply voltage headroom in the bandgap circuitry, and further does not significantly increase power consumption, thus being particularly applicable in low power, low voltage applications. The invention thus represents an advancement over conventional bandgap reference circuits, finding utility in wireless communications and portable computational devices, as well as any circuitry where stable reference voltages are needed.
One aspect of the present invention involves a bandgap circuit comprising two circuit branches electrically connected between a supply voltage and the ground. The bandgap circuit comprises an input circuit such as having two bipolar devices, three resistors, an amplifier, a ripple rejection circuit and a current mirror circuit. Through the amplifier, two bipolar devices generate a xcex94VBE signal, which is sensed by a resistor. The output of the amplifier is connected to the ripple rejection circuit, which may comprise a subtractor. The output of the ripple rejection circuit is the subtraction or difference of a supply voltage and the amplifier output voltage. The output of the ripple rejection circuit, which is connected to a pair of current mirrors, operates to modulate the currents through two circuit branches in bandgap circuit and keep these current the same.
The invention thus provides a bandgap circuit comprising two circuit branches electrically connected between a supply voltage and a ground, with an input circuit, an amplifier, a ripple rejection circuit, and a mirroring circuit. The input circuit provides input voltages at first and second input voltage nodes, for example, which may represent first and second base-emitter voltages associated with first and second diode-connected transistors having different emitter sizes. The amplifier senses the input voltages and provides a first control signal. The ripple rejection circuit provides a second control signal representative of a difference between the supply voltage and the first control signal, and the mirroring circuit provides currents to the circuit branches according to the second control signal, wherein the first current provides the reference voltage at a reference voltage node in the first circuit branch. In one implementation, the mirroring circuit comprises first and second PMOS transistors connected in the first and second circuit branches, respectively, and the ripple rejection circuit comprises a subtractor including two NMOS transistors providing the second control signal representing the difference between the supply voltage and the first control signal.
The PMOS transistors, in turn, receive the second control signal from the NMOS transistors and accordingly provide the currents to the circuit branches. For example, one NMOS transistor may comprise a drain terminal connected to the supply voltage, a gate terminal connected to the supply voltage, and a source terminal connected to the mirroring circuit, and the other NMOS transistor may comprise a drain terminal connected to the source terminal of the first NMOS transistor, a gate terminal connected to the amplifier output terminal, and a source terminal connected to ground. In this manner, the second control signal provided to the PMOS gates is the difference between the supply voltage and the first control signal, by which the currents provided by the PMOS transistors will not be affected by power supply ripple or noise.
Another aspect of the invention provides a system for reducing output ripple voltages in a bandgap voltage reference circuit. The system comprises a first MOS transistor with a drain and a gate connected to a supply voltage, and a source providing a control signal to a mirroring circuit. A second MOS transistor is provided, which comprises a drain connected to the source of the first MOS transistor, a gate terminal connected to an amplifier, and a source connected to ground. The gate of the second MOS transistor receives the amplifier output signal. The system produces the control signal representing a difference between the supply voltage and the amplifier output signal, by which the effects of supply voltage ripple or other power source noise is not transferred to the bandgap reference output.
Yet another aspect of the present invention provides a method of reducing ripple voltage in a bandgap voltage reference system. The method provides a feedforward supply voltage to the control terminal of the current mirror through a summation circuit, such as a subtractor. Thus, when the input terminals of the current mirror varies with supply voltage, the control terminals of the current mirror are adjusted in the same direction. As a result, the current in two branches of the bandgap circuit are insensitive to supply noise.
The provision of the control signal to the current mirrors of the bandgap circuit may be accomplished in a variety of ways. In one implementation, the second control signal is provided by connecting the drain and gate of a first MOS transistor to the supply voltage, connecting the source of the first MOS transistor and the drain of a second MOS transistor to a mirroring circuit in the system, connecting the gate of the second MOS transistor to receive the control signal from the amplifier, connecting the source of the second transistor to ground, and providing the control signal at the source of the first MOS transistor and the drain of a second MOS transistor representing a difference between the supply voltage and the first control signal.
To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which the principles of the invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.